Most frequent lesions and samples for laboratory diagnosis.

Marek’s Disease (MD) is a neoplastic disease of poultry caused by oncogenic alphaherpesvirus infection. Marek’s disease virus (MDV) has a very ubiquitous distribution and is highly resistant to environmental conditions therefore virtually all birds are exposed to strains of MDV during their life, and therefore are infected. But only in a few cases the disease will develop. The infection that occurs in birds is persistent and can be infected for life. The virus infects  lymphocyte cells which in some cases will be transformed into tumor cells and infiltrate different organs and tissues of the animal.

Marek’s Disease can occur with different clinical presentations:

 

MAREK’S DISEASE CLINICAL FORMS

Nervous Usually unilateral flaccid paralysis of the legs or wings is observed. In this case, tumor cells infiltrate  the peripheral nervous system, especially the sciatic nerve (neurolymphomatosis).
Visceral Mortality drip and sometimes diarrhea. At necropsy the presence of tumors in different organs is revealed. The most common are liver, proventriculus and spleen.
Flaccid neck Transient flaccid neck paralysis is observed on the birds. Forcing them to lay with their necks stretched on the floor. Usually associated with infection with highly virulent strains (“very virulent plus”) MDV (vv+ MDV).

Given the ubiquity and strength of the virus, vaccination is usually performed on the 1st day of life in hatchery to prevent the infection of the chicks. However, the effectiveness of the vaccine depends on various factors:

 

FACTORS INFLUENCING VACCINE EFFICACY

Pathotype Different strains of Marek exist: the virulent strains (“virulent” – v MDV), the highly virulent (“very virulent” – vv MDV) and highly virulent plus (“very virulent plus” – vv+ MDV). Among other things strains  vv+ MDV and vvMDV are characterized by being able to break the immunity generated by Marek vaccines HVT and bivalent . Here we do not know what field pathotypes are present  but the flaccid neck clinic observed on some farms is associated with vv+ MDV strains,  therefore probably these strains are circulating.
Serotype Also there are different serotypes of Marek (1, 2 and 3). Most common vaccines are  HVT (serotype 3) or Rispens CVI988 (serotype 1) and there are some combined vaccines. CVI988 vaccine seems that yields best protection against vv+ MDV. The HVT gives good protection against virulent strains, and in combination with strains of serotype 2 strains against vvMDV.
Genetics There is also genetic genetic effect: some strains are more susceptible to Marek others.
Conservation Marek’s vaccine is stored frozen in liquid nitrogen and its management in hatchery must be very good because the title of the vaccine drops very quickly.
Regime To further complicate the issue, since birds are  nowadays slaughtered very soon, in many cases they have no time to develop disease, and it is common not to vaccinate full dose. In fact most of the Marek cases seen in  slaughterhouse are usually in slower growing birds.

ANTEMORTEM: The signs depend on the clinical condition of the animals, paralysis of the limbs or neck flaccid paralysis can be seen.

POSTMORTEM: Lesions can include thickening, usually unilateral, of the sciatic nerve, nodular lesions in the viscera, thickening of the proventriculus wall, splenomegaly and the presence of perifollicular skin nodules.

LABORATORY DIAGNOSIS: MD diagnosis should not only confirm the disease but must rule out other birds neoplastic syndromes such as avian leukosis. This can be done by a histopathological study; therefore tissues to be sent (send samples in formalin) should include those that allow us to distinguish between the two diseases. Samples: sciatic nerve, skin, brain and bursa. Also send tissues or organs that are affected (liver, spleen, proventriculus, etc).

Image characteristic of a chicken neck with flaccid paralysis.

Image characteristic of a chicken neck with flaccid paralysis.

Limb paralysis.

Limb paralysis.

Images of perifollicular nodules in the limbs.

Images of perifollicular nodules in the limbs.

Images of perifollicular nodules in the neck.

Images of perifollicular nodules in the neck.

Splenomegaly.

Splenomegaly.

Proventriculitis.

Proventriculitis.

Lymphoid neoplastic infiltrate in the liver.

Lymphoid neoplastic infiltrate in the liver.